The present invention pertains to high gain, high frequency radio frequency (RF) multistage power amplifiers, e.g., used in wireless communication systems.
Electronic power devices, for example power transistors, characteristically operate at high current and/or high voltage. These high currents and voltages require that unique considerations be given to the physical design of the devices and their integration into a system. In particular, the higher the power, the greater the affect of electrical lead elements, i.e., the inherent resistive and reactive characteristics of electrical wires and connections, on the performance of the device. For example, high currents carried by power devices may be directed to the common ground path and, thus, may impact the physical arrangement of the power devices to ensure proper grounding for correct circuit operation and circuit stability. In general, the lower the resistance of the electrical paths connecting the power devices to the reference ground terminal, the more accurate and stable the circuit operation.
For high gain, high frequency applications, laterally diffused metal oxide semiconductors (xe2x80x9cLDMOSxe2x80x9d) power transistors have been preferred for forming gain stages in multistage amplifiers. LDMOS transistors have their common element (source) terminal formed on the underside of the transistor die (or xe2x80x9cchipxe2x80x9d). As such, the transistor source terminals may be directly connected to a common reference lead (e.g., a layer of conductive metal such as gold) formed on a substrate (such as a tungsten or ceramic thermal sink) to which the transistor chip is attached by using known die attach techniques. The LDMOS transistor input (gate) and output (drain) terminals are located on the topside of the transistor chip, and are commonly connected to other circuit elements of the amplifier using bond wires.
In a multistage amplifier, the current flowing through the respective transistor sources is combined in the common reference lead, forming a xe2x80x9ccommon lead current.xe2x80x9d The common reference lead current path has inherent (parasitic) resistive and reactive elements, which impact on the stability and operational performance of a conventional multistage amplifier. Because of the relatively high common lead current, the inherent resistance and reactance of the common reference lead may substantially affect the operational performance and stability of a multistage amplifier, even though this inherent resistance and reactance of the common reference lead is relatively small.
The xe2x80x9csharingxe2x80x9d of a common reference lead by sources of component transistors in multistage amplifiers leads to the problem of isolation degradation between gain stages. Over the years, the general trend has been to make electronic devices, such as cellular telephones, smaller and smaller. As the result, the stages of gain in multistage power amplifiers used in these electronic devices are also being moved closer and closer together. Combined with the presence of inherent resistive and reactive elements in the common reference lead, a feedback voltage results. Consequently, there is an overall isolation degradation between the gain stages of the multistage amplifier, resulting in instability and reduced operational performance.
For purposes of better illustration, FIG. 1 illustrates a physical packaging of a conventional multistage amplifier 100, employing LDMOS transistors for the respective gain stages, and also showing the common lead current paths. FIG. 2 is a schematic illustration of amplifier 100, showing the inherent resistance and inductive reactance of the common reference lead connecting the respective source terminals of the gain stage transistors to reference ground.
The amplifier 100 includes a thermally conductive substrate 140 used as both a heat sink and support structure. The substrate 140 is covered with an electrically conductive material, such as gold, forming an electrically conductive layer 130. A pair of LDMOS power transistor chips 110 and 120 are directly attached to the electrically conductive layer 130, with the respective underlying source terminals of the transistors are directly connected to the conductive layer 130. The transistors 110 and 120 are electrically connected in cascade, with transistor 110 representing the first gain stage, and transistor 120 representing the second gain stage. Although transistors 110 and 120 are illustrated as single transistors, it will be understood by those skilled in the art that each transistor 110 and 120 may actually comprise of two or more physically separate power transistors operating in parallel.
The first gain stage transistor 110 has its top-side gate terminal connected to a gate lead 165 by one or more bond wires 170. The gate lead 165 is attached to (or formed on) the substrate 140, electrically isolated from the conductive layer 130. The gate lead 165 is connected to a source generator 180. The source generator 180 is external to the amplifier device 100, and forms a circuit between the gate lead 165 and the chassis ground 150. Resistance through the source generator 180 is represented by resistor 132.
The top-side drain terminal of transistor 110 is connected to the top-side gate terminal of the second gain stage transistor 120 by one or more bond wires 174. The top-side drain terminal of transistor 120 is connected to a drain lead 163 by one or more further bond wires 172. The drain lead 163 is attached to (or formed on) the substrate 140, electrically isolated from the conductive layer 130. The drain lead 163 is connected to a load, shown as a resistor 182. This resistor 182 is also external to the amplifier device 100.
The underlying side of substrate 140 is attached to a ground plane, e.g., a chassis ground/heat sink 150, with the electrically conductive layer 130 in direct contact with the chassis ground 150. In this manner, the electrically conductive material 130 acts as the common reference lead for the source terminals of transistors 110 and 120, providing a current path from the respective transistor source terminals to chassis ground 150.
In particular, the electrical currents from the respective transistor source terminals combine to form a common lead current (shown by arrows 190), which must contend with the inherent (parasitic) resistance and inductive reactance in the electrically conductive layer 130 (common lead). As a result, the amplifier 100 will generally be unstable and suffer from isolation degradation between its gain stages due, in part, to the close proximity of the transistors 110 and 220 and the inherent resistive and inductive reactance characteristics of the electrically conductive layer 130. Resistors 150, 152, 154, 156, 158 and 160 represent the inherent resistance, and inductors 151, 153, 155, 157, 159 and 161 the inductive reactance, respectively, of the common reference lead current path connecting the transistor source terminals to reference ground. The respective source terminals of transistors 110 and 120 are electrically connected to each other through the respective inherent resistors 152, 154, 156 and 159, and inductors 153, 155, 157 and 159 in the common lead current path. Notably, the inductors are in parallel with the resistors and their inductive reactance is at least equal to the skin resistance of the conductive layer 130 at any given frequency.
Although the inherent resistive and inductive elements may represents very small values, they may cause significant performance variations because of the gains of the individual stages. Further, the resistive and inductive characteristics of the conductive layer can make the current flowing from the source terminal of one transistor flow towards the source terminal of the other transistor. As a result of these factors, a conventional multistage amplifier as illustrated in FIGS. 1 and 2, tends to be unstable and suffers gain stage isolation degradation.
Various approaches have been applied to improve gain stage isolation and to improve the stability of multistage amplifiers. One approach has been to move the active devices (i.e., the power transistors) closer to the ground terminal (i.e., the chassis ground), so that feedback voltage is reduced. For example, when power transistors, such as LDMOS transistors, are placed on supporting or thermally conductive material as part of a power amplifying circuitry, via holes filled or plated with electrically conductive material may be formed in the supporting or thermally conductive material. The via holes provide additional multiple current paths from the common reference lead to the ground or chassis ground, which is generally located on the backside of the supporting or thermally conductive material. This approach brings the ground physically closer to the active device, thereby reducing the effects of the inherent (parasitic) elements. Unfortunately this approach also has certain drawbacks, such as relatively high mechanical and reliability costs of the amplifier package.
Another approach has been to use resistive xe2x80x9cdissipativexe2x80x9d loading of the gain stages. However, the downside of using dissipative loading is that gain and efficiency are reduced. Thus, an alternative approach that provides a reliable and mechanically cost effective means of power amplification remains highly desirable.
A multistage amplifier comprises two or more power transistors configured to produce superior isolation between gain stages by providing an alternative current path to ground for at least one of the transistor sources, resulting in increased stability and improved operational performance of the amplifier.
In one embodiment, two power transistors are attached to a common mounting substrate and electrically connected in series in a multistage gain configuration. The transistors may be formed on the same, or separate, semiconductor chips attached to the substrate. The substrate is covered by a layer of electrically conductive material, such as gold, which acts as a common reference lead. A source generator is attached at one end of the substrate and is connected to the input terminal of a first transistor. The output terminal of the first transistor is connected to the input terminal of the second transistor. The output terminal of the second transistor is connected to a load resistance, thereby completing the amplifier circuit.
In accordance with the invention, the common element terminal of the first transistor is isolated from direct electrical connection to the conductive layer (common element lead) and is instead connected by one or more bond wires through the topside of the transistor to the conductive layer at a location near the input of the source generator and distant from the second transistor. The common element terminal of the second transistor is directly connected to the conductive layer (common reference lead) at the bottom of the transistor, in a conventional fashion.
Connecting the respective transistor common element terminals at relatively distant points on the common reference lead results in distinct ground current paths for each transistor, which decreases the total current flow at any one point in the common element lead. This, in turn, increases the mutual resistance and inductive reactance between the gain stages, and provides increased stability for the amplifier device.
Other and further aspects and advantages of the invention will become apparent in view of the following detailed description of the preferred embodiment.